An occupancy sensor is designed to detect the presence of a person(s) in a room, usually in order to determine whether various electrically powered loads in that room (e.g., lights, ventilation, and the like) should be turned on or not. This is of particular advantage to institutions that have occupants who are not directly responsible for paying for the electricity they consume, since these people often do not exercise diligence in regularly turning off electrically powered loads such as lights, ventilation, and the like, when they leave a room. Occupancy sensors may therefore provide a means to conserve a significant amount of energy. This has led many businesses to purchase occupancy sensors voluntarily. This potential for energy savings has also resulted in laws being passed in certain states that mandate the use of occupancy sensors in large areas as an environmental conservation measure.
Some state and local energy conservation/building codes require installation of two light switches in the construction or reconstruction of offices, each to control a different portion of the overhead lighting. The reasoning behind such a requirement is that, in the interest of energy conservation, employees and janitorial personnel have the opportunity to use approximately one half of the light they would normally require in their day-to-day activities. Depending upon the amount of ambient light available, employees working in a room may select to use only one half of the available bank or banks of lights.
Further, employees may customize their specific lighting needs to their activities and location in the room. For example, employees working in an area not receiving sufficient ambient light may require more artificial light, depending upon their specific activities. Similarly, employees located in an area receiving sufficient ambient light may require less artificial light. Utilizing office lighting effectively (e.g., using only one-half of the available lighting, and using lighting only in occupied offices) results in substantial energy savings. In addition, for computer applications, it is advantageous to reduce the level of light to eliminate the glare on cathode ray tubes (CRT).
Conventional manual switches are inefficient because they depend upon human judgment to turn all or only a portion of the lights on and off. Existing automatic wall switches are more efficient, but still make errors, and have less than optimal sensitivity.
Commercially available occupancy sensors and load control units have been designed to replace existing wall switches in commercial and private applications. These units typically include load switching devices that replace the mechanical switch contacts found in a manually-operated switch. These load switching devices may include relays, SCRs, Triacs, transistors, or other electrical load switching devices that may be controlled by power control circuitry including, for example, a programmable controller, or the like. Many of these replacement units require a power supply for the power control circuitry that must supply power to the control circuitry whether or not the load switching device is in the on-state or the off-state. The wiring that exists in the existing switch enclosures, the mechanical constraints imposed by the existing switch enclosures, and the constraints presented by the existing loads cannot be easily altered and must be tolerated by the unit that is replacing the existing switch.
Units that have been designed as replacement devices for existing switches range from simple dimmer switches to intelligent lighting systems with microprocessor control. Commercially viable replacement units for business or residential locations are preferably low cost, robust, small in size, meet stringent safety considerations, as well as have low electrical power dissipation, and attractive physical features. In addition, the replacement unit's contacts preferably emulate the simple mechanical air gap switch it replaced. The replacement unit should also have a similar voltage drop when the contacts are closed, essentially zero leakage current through the contacts when the contacts are open, and guarantee safety from hazardous voltages when the contacts are open. Underwriters Laboratories (UL), the National Electrical Manufacturers Association (NEMA), the National Electrical Code (NEC), and other electrical safety organizations and documents, generally agree that 0.5 milliamperes (mA) of electrical current may pass through the human body without creating hazard of electrical shock. This current level has been established as a safety standard for electrical current leakage that may incidentally occur in an electrical device. Various manufacturers of automatic wall switch type devices have utilized this allowable leakage current as a power source for the device.
A number of load control devices and/or power supplies for use with load control devices are described in commonly-assigned U.S. Pat. Nos. 5,821,642, 6,307,354, 6,466,826 and 6,472,853 to Nishihira et al, U.S. Pat. No. 5,774,322 to Walter et al, U.S. Pat. Nos. 5,777,837 and 5,856,905 to Eckel et al, and U.S. Pat. No. 6,262,565 to Williams et al, which are each hereby incorporated herein by reference. U.S. Pat. No. 6,262,565, to Williams et al, provides a power system for an electrical load switch that replaces the simple mechanical contacts of a wall switch with those elements necessary to power control circuitry, provide controllable contacts, ensure thermal stability in a wall switch enclosure, control the off-state leakage current to ensure safe and robust operation of sensitive loads, and provide a safety device to guarantee that a no leakage off-state exists to protect a maintenance person from voltage potential with respect to neutral during load replacement. As with many power systems used with existing occupancy sensors, however, the power system disclosed in U.S. Pat. No. 6,262,565 employs a power supply in series with the load. This is often advantageous in retrofit situations where the sensor power supply and relay are connected into existing lighting circuits in the most expedient way, as a replacement for the manual wall switch in the portion of the circuit already switched at the wall.
These types of power supplies are disadvantageous because they allow a small amount of current to flow in the load in off state, and that current may cause malfunction of certain electronic lamp ballasts. These types of power supplies are also disadvantageous because they require a minimum amount of current flow to function with the load in the on-state, and because it is difficult for a single design to accommodate a large range of load current levels. Active sensing requires power to transmit. A need therefore exists for an improved power supply for occupancy sensors that does not require a minimum load but can provide enough power for an active motion sensor. A need also exits for a system that physically fits in the space allotted for a wall switch without the power supply negatively affecting the motion sensor.